The invention is an improved method for diffusion bonding of SiC ceramics.
The diffusion bonding of a metal foil layer between two ceramic layers (such as SiC (silicon carbide), or ZrC (zirconium carbide) or B4C (boron carbide)) has an established history. Unfortunately, conventional metal diffusion bonding, which uses vacuum hot pressing, results in numerous difficulties. A five hour heat treatment in a vacuum (pressure less than 1xc3x97106 torr) at temperatures of 1200xc2x0 C. to 1500xc2x0 C. and applied pressure of 500 to 2,000 psi is required to produce bonding. The equipment needed for conventional vacuum hot pressing is expensive to purchase and expensive to operate. Also, conventional metal foil diffusion of this type is a single unit manufacturing that is slow, not conducive to mass production, and hence prohibitively costly.
The use of metal foils to achieve diffusion bonding between ceramics, or a ceramic and a metal, is disclosed in U.S. Pat. No. 2,857,663. Oxide ceramics such as Al2O3 are disclosed, and diffusion bonding is effected by melting the metal foil insert. U.S. Pat. No. 4,624,403 discloses the use of various metal foils to bond SiC ceramics upon application of pressure and temperature in a vacuum or inert atmosphere. Unlike the ""663 process, which melts the foil, the ""403 process promotes diffusion bonding without melting the metal foil. The process of U.S. Pat. No. 5,125,557 also uses an inert or vacuum atmosphere furnace to bond ceramics with various metals. U.S. Pat. No. 5,599,468 describes the use of a pulsed electron radiation heat source to join ceramics such as SiC to SiC or other metals by the use of metal foil inserts.
Accordingly, a need remains for a process for diffusion bonding metal foils or other metal inserts between ceramic layers in a process suitable for mass production of less expensive, higher quality products than those resulting from single unit production.
In order to meet this need, the present method uses a defocused laser to heat and to join ceramics with the use of a thin metal foil insert. The rapid, intense heating of the ceramic/metal/ceramic sandwiches using the defocused laser beam results in diffusive conversion of the refractory metal foil into a ceramic and in turn creates a strong bond therein. Most preferably, the foils are rhenium or molybdenum. The foil thickness can be 12-100 xcexcm, most preferably between about 12 and 25 xcexcm. The process is ordinarily conducted in a glove box containing an inert atmosphere, preferably argon (with less than 1 part per million each of oxygen, nitrogen and moisture).